A fuel cell is a device which converts chemical energy directly into electrical energy. A typical fuel cell comprises two electrodes, a cathode and an anode, separated by a matrix impregnated with an electrolyte, an external electrical connection between the anode and the cathode, and manifolds and ducts for supplying reactants to the fuel cell and for removing reaction products from the fuel cell. One common electrolyte is phosphoric acid. Fuel cells which use phosphoric acid as the electrolyte are referred to as phosphoric acid fuel cells.
A fuel cell generates electricity when a hydrogen-rich gas is supplied to the anode as a fuel and oxygen, typically in the form of air, is supplied, to the cathode as an oxidant. The anode oxidizes hydrogen to hydrogen ions and electrons. The hydrogen ions flow through the electrolyte to the cathode, while the electrons flow through the external electrical connection to the cathode. At the cathode, oxygen is reduced and reacted with the hydrogen ions and electrons to produce water and heat. Phosphoric acid fuel cells typically operate at a temperature between 150.degree. C. and 200.degree. C. and a pressure between 1 atmosphere and 8 atmospheres.
At typical phosphoric acid fuel cell operating conditions, phosphoric acid is extremely corrosive. Although the phosphoric acid in the fuel cell is normally contained within a matrix fabricated from a corrosion-resistant material, it is not uncommon for some phosphoric acid to escape entrainment in the matrix and come into contact with the manifolds and ducts. Unless the manifolds and ducts, which are typically fabricated from a metal such as steel, are protected in some way from the phosphoric acid, they will corrode. Corrosion of fuel cell manifolds and ducts can lead to a loss of integrity such that phosphoric acid and fuel cell reactants are released to the surrounding environment, forcing the fuel cell to be shut down.
Several means are available to inhibit corrosion of fuel cell manifolds and ducts. One method is to fabricate the manifolds and ducts from materials which inherently resist corrosion. Such materials include stainless steel and other metal alloys. Other corrosion-resistant materials include plastics, such as high temperature reinforced polysulfones. However, these corrosion-resistant materials are generally expensive, making them impractical for many fuel cell applications. Moreover, many of these corrosion-resistant materials, especially plastics, are difficult to fabricate into fuel cell manifolds and ducts.
An alternate approach to protecting metal phosphoric acid-fuel cell manifolds and ducts from corrosion is to coat the manifolds and ducts with a corrosion-resistant compound. The preferred coatings are fluorinated hydrocarbon polymers. The most preferred corrosion-resistant coating for phosphoric acid fuel cell manifolds and ducts is perfluoroalkoxy (PFA) polymer. While PFA and similar polymers provide satisfactory corrosion protection, it is difficult to apply the coatings directly to the surfaces of the manifolds and ducts because PFA and similar compounds are chemically inert. Generally, these compounds can be applied to metal surfaces only if a bonding agent such as an organosilane primer is used to aid adhesion.
Even after the polymer has been successfully applied to a metal surface, it is difficult to keep the coating intact. When placed in service, hydrogen molecules and water vapor penetrate the polymer coating and break the bonds between the metal surface and the polymer. This causes the polymer to slough off of the metal surface, exposing the metal surface to corrosive attack by phosphoric acid. Moreover, the polymer which sloughs off of the metal surface interferes with the flow of reactants and products through the manifolds and ducts, forcing the fuel cell to shut down.
One way to minimize the effects of gas or water vapor penetration is to apply the polymer in a thick coat. However, a thick coat of polymer accentuates the differences between the coefficients of thermal expansion of the polymer coating and the underlying metal surface. For example, PFA has a coefficient of thermal expansion which is about three times that of steel. When reactants are supplied to a cold fuel cell and the fuel cell is heated to operating temperature, the polymer coating expands to a much greater extent than the metal surface, causing the polymer to debond from the metal surface of slough off. A similar phenomenon occurs when the supply of reactants is terminated and the fuel cell is cooled. As a result, the metal surface is exposed to corrosive attack by phosphoric acid, while the debonded polymer coating interferes with the fuel cell operation.
The problem associated with chemically bonding PFA and similar compounds to fuel cell manifolds and ducts can be overcome by mechanically bonding the coatings to the metal surface. A mechanical bond between the polymer coating and the metal surface is formed when the polymer is applied to a porous metal surface, or tie coat, which has been bonded to the top of the manifolds and ducts. The tie coat contains numerous pores and cracks. The mechanical bond between the polymer coating and the metal tie coat is formed when molten polymer flows into the pores and cracks of the tie coat and solidifies.
Several techniques are available for mechanically bonding a polymer coating to fuel cell manifolds and ducts. The most common method is to spray molten metal onto the surface of the manifolds and ducts with a thermal or plasma spray gun in order to form the porous tie coat. The polymer may be applied to the porous tie coat either as a solid or in a semi-fluid state. Upon heating, the polymer flows and fills the pores and cracks of the tie coat. When cooled, the polymer solidifies and becomes locked inside of the pores and cracks of the tie coat, forming the mechanical bond. While this method can be successfully used to apply a corrosion-resistant polymer coating to fuel cell manifolds and ducts, alternate methods of producing a comparable coating are desirable in order to permit manufacturing operations to be optimized.
Accordingly, there has been a continuous effort in this field of art to develop an alternate means for producing a corrosion-resistant polymer coating on top of fuel cell manifolds and ducts.